The FBI is investigating a possible financial link between the Saudi royal family and the 11 September hijackers, in an inquiry that the Bush administration has attempted to keep secret for political reasons, it was reported yesterday.

The investigation was triggered by the discovery of money transfers from a US account under the name of Princess Haifa al-Faisal, wife of the Saudi ambassador to Washington, to two Saudi students in California who in turn gave financial assistance to two of the hijackers soon after they arrived in the US.

The inquiry has been under way for some time and congressional investigators have been putting pressure on the White House to release details of its findings.

Some in Congress believe the investigation was being kept secret so as not to alienate the Saudis at a time when Washington needs its help for a conflict with Iraq.

According to Newsweek, monthly payments of $3,500 were sent from Princess Haifa's account in Washington to the family of Omar al-Bayoumi, a student in San Diego, starting in early 2000. Earlier al-Bayoumi had befriended Khalid Almidhar and Nawaf Alhazmi, who had just arrived from an al-Qaeda meeting in Malaysia and who would later crash a hijacked airliner into the Pentagon.

Al-Bayoumi is reported to have thrown a welcome party, guaranteed the lease on a flat next door to his own and paid their first two months' rent.

Al-Bayoumi left the US in July 2001 and went to Britain to study business at Aston University in Birmingham. British police picked him up for questioning, but he denied any prior knowledge of the attack and was released a week later. He is believed to be back in Saudi Arabia. After Al-Bayoumi left the US, the monthly payments went to another student, Osama Basnan, who had also made friends with the two future hijackers and who later celebrated the 11 September attacks at a party.

A spokeswoman for the princess said she had not been questioned about the transfers and would 'co-operate fully'. There is no evidence she knew the students' intentions.